Blood purifiers called dialyzers are used for dialysis therapy. A dialyzer includes a casing that houses a bundle of hollow fiber membranes. The casing includes four ports: a blood inflow port for introducing blood extracted from a shunt forming in a dialysis patient, for example, a blood outflow port for reinfusing blood into the patient, and dialysate inflow/outflow ports for inflow and outflow of the dialysate.
For dialysis therapy, blood is extracted out of the body (blood extraction) and conveyed to the dialyzer. Blood flows into the dialyzer via the blood inflow port, flows through hollow fiber membranes and out from the outflow port, and is returned to the body (reinfusion of blood). In addition, dialysate is supplied into the casing via the dialysate inflow/outflow port to fill between the hollow fiber membranes. The blood and dialysate undergo substance exchange via the hollow fiber membranes.
Further, in order to substitute for the function of the kidney to adjust the water content, the dialyzer is used to perform fluid removal for discharging excessive water content out of the body in the dialysis therapy. In the fluid removal process, the quantity of flow of dialysate is controlled to increase the quantity of outflow of dialysate compared to the quantity of inflow of dialysate, so that negative pressure is generated within the casing to thereby extract the water content within the blood toward the dialysate (i.e., by ultrafiltration).
Because an excessive quantity of fluid removal applies a load on the dialysis patient, it is necessary to measure the quantity of fluid removal accurately. Patent Document 1 describes obtaining a difference between measurements by a flowmeter disposed in the dialysate inflow channel and measurements by a flowmeter disposed in the dialysate outflow channel to thereby calculate the quantity of fluid removal.
In general, the allowable range of error in fluid removal is extremely limited, and positive displacement flowmeters such as oval flowmeters and Roots type flowmeters are used for such a requirement for high accuracy. The positive displacement flowmeter is configured to transfer liquid via a measurement chamber. Because the volume of the measurement chamber is known, the quantity of flow can be measured with high accuracy.
While the positive displacement flowmeters provide highly accurate measurement, it is technically difficult to cause the positive displacement flowmeter disposed close to the dialysate inflow channel and the positive displacement flowmeter disposed close to the dialysate outflow channel to output completely identical measurements, because each measurement value contains a measurement error based on the individual difference. It is therefore necessary to perform calibration in order to eliminate the measurement error between these two flowmeters. More specifically, a reference flowmeter capable of measuring the volume of liquid which is actually supplied accurately (at least to a degree which satisfies the required accuracy) is used to correct each of the measurements of the two flowmeters.